1. Field of the Invention
This invention relates to an image forming system and a control method thereof and in particular, to an image forming system for developing an electrostatic latent image formed on a photo sensitive body of an electrophotographic copier, a facsimile, a laser printer, or the like, with toner and transferring the toner image to recording paper by applying a voltage to a transfer roll wherein a high-quality image can be formed by appropriately controlling a transfer bias applied to the transfer roll, and a control method of the image forming system.
2. Description of the Related Art
Hitherto, an electrophotographic copier, a facsimile, a laser printer, and the like have been known as an image forming system which comprises an image carrier such as a photosensitive drum and transfer means such as a transfer roll for pressing against the image carrier, makes a transfer material pass through the space between the image carrier and the transfer roll pressing there against, and at this time, applies a transfer bias to the transfer means for transferring a toner image on the image carrier to the transfer material such as recording paper.
In the image forming systems, a configuration for applying a constant current or a constant voltage to a transfer roll which is transfer means just before print, estimating a resistance value of the transfer roll from the current or voltage value at that time, which will be hereinafter referred to as BTR resistance monitor, and controlling the transfer bias applied to the transfer roll which is the transfer means to the optimum value based on the BTR resistance monitor is proposed, for example, as disclosed in Japanese Patent Unexamined Publication Nos. Hei. 3-157681 and Hei. 5-297740.
The BTR resistance monitor is configured so as to determine the optimum transfer bias applied to the transfer roll using a table provided for storing the optimum transfer bias corresponding to current value A, for example, if the current value A is measured when a constant voltage is applied to the transfer roll.
By the way, generally, urethane foam is used as the transfer roll material and an ionic conductive agent is contained in the urethane foam, thereby providing conductive performance or carbon particles are dispersed in the urethane foam, thereby providing conductive performance.
Particularly, the resistance value of the transfer roll of the type wherein an ionic conductive agent is contained, which will be hereinafter referred to as ion transfer roll, changes in the order of 10.sup.6 to 10.sup.10 between a high-temperature environment (28.degree. C./85% RH) and a low-temperature environment (10.degree. C./15% RH).
Therefore, for example, when such a transfer roll is used and constant voltage control is performed for transferring an image, to obtain an appropriate transfer current voltage, it is necessary to apply a constant current or voltage to the transfer roll just before print, estimate the resistance value of the transfer roll, which will be hereinafter referred to as BTR resistance, from the current or voltage value at that time, and determine the optimum transfer bias.
To use the ion transfer roll for continuous print, the BTR resistance largely changes due to a BTR resistance energization rise or an in-machine temperature rise caused by heat from a fuser, or the like.
To cope with such a case, a configuration for applying a constant current or voltage to the transfer roll at the non-print time between pages in continuous print, estimating the BTR resistance from the current or voltage value at that time, and determining a proper transfer bias to the transfer roll based on the estimated resistance value is also proposed as disclosed in Japanese Patent Unexamined Publication Nos. Hei. 3-157681 and Hei. 5-297740.
Recording paper just after fusing is high in resistance and low in moisture content. Thus, in second-side transfer (rear-side print) at the double-sided print time, transfer image quality failure such as transfer memory or toner scatter easily occurs and the proper transfer bias range is narrow.
Therefore, the voltage latitude of transfer bias is narrow for one BTR resistance and fine control becomes necessary. For example, if a difference between the BTR resistance estimated by the BTR resistance monitor and the actual BTR resistance is large, transfer image quality failure occurs and its occurrence level becomes terrible.
The BTR resistance of the ion transfer roll depends largely on the environment; for example, in a high-temperature, high-humidity environment (28.degree. C./85% RH), the resistance value at the 1000-V applying time is about 1.0.times.10.sup.7 ohms and in a low-temperature, low-humidity environment (10.degree. C./15% RH), the BTR resistance rises to about 1.0.times.10.sup.9 ohms.
If an attempt is made to perform constant current control of the BTR resistance of the transfer roll having such characteristics with one constant current value and estimate the BTR resistance from the voltage value at that time, voltage value change with BTR resistance change becomes large in the low-temperature, low-humidity environment and becomes small in the high-temperature, high-humidity environment; the BTR resistance estimating accuracy becomes low in the high-temperature, high-humidity environment.
In contrast, if an attempt is made to perform constant voltage control of the BTR resistance with one constant voltage value and estimate the BTR resistance from the current value at that time, voltage value change with BTR resistance change becomes large in the high-temperature, high-humidity environment and becomes small in the low-temperature, low-humidity environment; the BTR resistance estimating accuracy becomes low in the low-temperature, low-humidity environment.
To solve such a problem, for example, a configuration for applying a constant voltage or current at least two times each in a different value and estimating the current-voltage characteristic of the BTR resistance at that time, and determining the optimum transfer voltage based on the estimation is also proposed as disclosed in Japanese Patent Unexamined Publication No. Hei. 3-157681.
Recording paper after second-side transfer, which undergoes twice transfer and once fusing, is strongly charged and curls after the fusing. Therefore, the recording paper after the second-side transfer is easily attracted to the image carrier electrostatically and mechanically, and a peel failure from the image carrier after transfer easily occurs particularly in a high-temperature, high-humidity environment.
By the way, in recent years, with faster desktop laser printers, the laser printers have been provided each with a large-capacity recording paper tray or the paper feed speed has been made higher (process speed 90 mm/sec or more) for enhancing productivity. Further, means for narrowing the non-print timing between one print and the next or the like has been used for enhancing productivity of the laser printers. Thus, sufficient BTR resistance monitor time between pages may not be taken.
Furthermore, the paper feed speed varies depending on the recording paper type. To use recording paper with large transport resistance, the transport speed of the recording paper is slow as compared with that of any other recording paper. At this time, if the recording paper is passing through a transfer nip portion at the timing of applying a transfer bias of a constant voltage or current to estimate the transfer roll resistance between pages, the BTR resistance cannot correctly be estimated. If the BTR resistance cannot correctly be estimated, the optimum transfer bias cannot be applied, causing transfer image quality trouble to occur.